def p(n):
    for i in range(2,n):
        if n%i==0:
            return False
    return True
    
x=int(input())

a=[]
for i in range(2, x+1):
    if p(i):
        a.append(i)
        
m=0
for i in a:
    if str(i) in str(x):
        m=i
print(m)
